The experiments proposed in the current application use well-established micro-pressure-sensor and laser Doppler vibrometer techniques to probe underlying cochlear mechanics relating to otoacoustic emissions (OAEs). This application aims to provide direct intracochlear observation in vivo to further understand the fundamental questions in OAE generation, propagation and cochlear signal processing in reverse transmission. The first aim will explore the cochlear active process in reverse transmission and the rest of the aims will continue and extend our previous work on the generation and propagation of distortion product OAEs (DPOAEs) in the cochlea. OAEs are sounds generated within the cochlea and being detected in the ear canal, carrying the information about the mechanisms that generate and shape them, thus providing a noninvasive view into cochlear mechanics. However, how to relate OAE to cochlea mechanics is a big challenge in OAE research. In the forward direction, sound causes a traveling wave along the cochlear partition, which is boosted by the outer hair cells' activity (cochlear amplifier). However, the role of cochlear amplifier and cochlear fluid in cochlear reverse transmission is not clear. With simultaneous intracochlear (the basilar membrane velocity and intracochlear pressure) and ear canal pressure measurements, aim one studies the generation sites of OAE in the cochlea and further explore multiple tones suppression in active cochleae; aim two is designed to quantify the traveling wave and compression wave pressures in both forward and reverse transmissions; aim three explores the cochlear active process in reverse transmission. Thus the results will provide fundamental intracochlear observations, test the OAE generation theories and form the foundation for developing three-dimensional cochlear model. Finally, results from the current application will lead to more efficient OAE hearing screening program used in research and clinic.